With senator Barack Obama poised this week to clinch his party's nomination for President, there are growing fears in some quarters that the Democratic party may not be choosing its strongest candidate to beat Republican John McCain.

Senator Hillary Clinton has been making that argument for weeks. Now some recent polls and analysis, looking particularly at vital battleground states and support among white voters, have bolstered her case - even as Obama looks certain to become the nominee.

Obama supporters reject this argument and point to his record of boosting Democratic voter turnout, especially among the young. But sceptics in the party, already nervous about nominating Obama after the furore over outspoken pastor Jeremiah Wright, are growing increasingly concerned. 'There is an element of buyer's remorse in some areas. The question is whether it gets really strong now or in September - or even after the election is over, if he loses,' said Steve Mitchell, head of political consultancy Mitchell Research.

Another boost to Clinton's case came late last week after a pro-Obama preacher gave a race-tinged rant against her at Obama's church in Chicago. In a recent sermon Michael Pfleger - a long-term Obama backer who is white - mocked Clinton as an entitled white person angry at a black man having beaten her. His angry, red-faced speech, in which he mimicked Clinton weeping, was played repeatedly across American cable channels and the internet.

The news sent shock waves through Democratic circles; many had hoped Obama had put 'pastor problems' behind him. 'It is more of the same problem as Wright. It reinforces the image among some voters that Obama does not share their values,' said Mitchell.

The uproar also lent a disturbingly antagonistic tone to scenes in Washington DC where Clinton and Obama supporters gathered yesterday outside a party rules meeting called to resolve the problem of the disputed Michigan and Florida primaries, which Clinton claims as victories. Clinton supporters chanted 'Count our votes!' and waved placards and banners. Clinton wants those states' delegations seated at the Denver convention, even though they broke party rules by holding early contests.

Obama is now to some extent limping to the finishing line. Clinton's refusal to bow out even though her odds of victory have become almost impossible has seen her win several of the most recent contests. In fact, since 4 March, Clinton has won around half a million more votes than Obama. That run of victories should easily continue today when Puerto Rico goes to the polls, and could even extend to the final primaries - South Dakota and Montana - which vote on Tuesday. Obama had been expected to win there, but Clinton has been campaigning furiously and it could be close.

Clinton has been making the case for several months, as her support has grown stronger among white working class voters, that those voters will not support Obama in a general election. By contrast, experts believe Obama's core - educated Democrats and blacks - will remain loyal to the party no matter who the nominee is. There is strong data to back that up, especially from recent votes in West Virginia and Kentucky where large proportions of Clinton voters said they would not back Obama in November.

There is also a growing fear that many of the women backing Clinton are turning against Obama. Clinton and her supporters have controversially accused their rival, and the media, of being misogynistic in the last few weeks of the race. A recent Pew Poll showed Obama's support among white women collapsing from 56 per cent to 43 per cent.

But the electoral fact remains the same. The dramatic Obama vs Clinton contest is now down to a few hundred uncommitted party 'superdelegates', who are under huge pressure to make their decisions in the next few days. Senior party figures, including House Speaker Nancy Pelosi, have been working privately to convince them to make that decision as soon as possible, ideally this week. With Obama's delegate lead unassailable, the vast majority are almost certain to come over to his side.

Republican analysts, meanwhile, are surprised about how healthy their party's prospects look in a year when almost all indicators suggested they should lose. McCain remains competitive against Obama. He even leads in some key states. Indeed, some research predicts he could romp home against Obama.

It is that prospect, Clinton supporters say, that leads them to keep fighting. They point to Obama's performance in North Carolina as a bellwether: it was his strong win there earlier this month that dealt an almost fatal blow to Clinton's chances. Yet, two weeks after that win, polls showed Clinton easily outperformed Obama there when measured against McCain. 'Clinton has a very strong argument that she is a stronger candidate against McCain. It is just that it has fallen on deaf ears,' said Mitchell.